Voice over Long Term Evolution (VoLTE) is a voice service based on an Internet Protocol multimedia subsystem (IMS). Because the IMS supports a plurality of accesses and abundant multimedia services, the IMS becomes a standard core network architecture in an all-IP era. The VoLTE is an IP data transmission technology. Instead of a 2nd generation mobile communications technology/3rd generation mobile communications technology (2G/3G) network, all services are carried on a 4th generation mobile communications technology (4G) network, so as to implement unification of data and voice services in one network. S8 interface home routing (S8HR) is a VoLTE roaming solution. FIG. 1 is a schematic diagram of a VoLTE S8HR architecture. When an S8HR terminal initiates IMS registration, a visited public land mobile network (VPLMN) selects a public data network gateway (PGW) in a home public land mobile network (HPLMN) for the terminal to establish a connection, and sends registration information to an IMS device in the HPLMN for registration.
However, single radio voice call continuity (SRVCC) in the S8HR roaming architecture is a technical problem difficult to resolve. The SRVCC means: If a terminal when performing a VoLTE service enters an area with poor Long Term Evolution (LTE) network coverage, and LTE signal quality in the area is not high enough to continuously support a VoLTE session requirement of a user, a network side triggers an SRVCC handover procedure to hand over the VoLTE session to a conventional 2G/3G circuit switched (CS) session to continue the conventional 2G/3G circuit switched session, ensuring that a voice conversation of the user is not interrupted. It is required that voice interrupt latency caused in an entire handover process cannot exceed 300 ms. However, in an S8HR roaming architecture, SRVCC handover cannot meet the requirement.